Allocation of uplink reference signals in a mobile communication system

ABSTRACT

A mobile communication system network node (NN) that serves user equipments (UEs) has fewer orthogonal reference signals (RSs) than a maximum number of UE antenna ports (APs) that can be served by the NN. A channel quality of a channel between the AP and the network node is ascertained for each of the APs. Whenever a number of APs of UEs served by the NN exceeds the number of RSs, all RSs are allocated to a subset of all of the APs by means of an allocation process such that: each RS is allocated to only one of the APs; each AP has no more than one RS allocated to it; and allocation decisions are a function of the channel qualities of the respective APs such that the higher the channel quality, the higher priority the corresponding AP is given as a candidate for receiving an RS allocation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/346,333 filed Dec. 30, 2008, the disclosure of which isfully incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to transmission of reference signals in anOrthogonal Frequency Division Multiplex (OFDM)-based system, and moreparticularly to transmission of demodulation reference signals in anOFDM-based communication system.

BACKGROUND OF THE INVENTION

In the Long-Term Evolution (LTE) mobile communication system defined bythe 3^(rd) Generation Partnership Project (3GPP), uplink radiotransmissions utilize Discrete Fourier Transform (DFT)-spread-OFDM(DFTS-OFDM) techniques. FIG. 1 a is a block diagram illustrating howDFTS-OFDM works. Blocks of M modulation symbols 101 are first applied toa size M DFT 103. The output of the DFT 103 is then supplied to afrequency mapper 105, which maps the DFT output to selective consecutiveinputs of a size N Inverse DFT 107 that can, for example, be implementedby means of Inverse Fast Fourier Transform (IFFT) processing. Byadjusting the block size M, the instantaneous bandwidth of thetransmitted signal can be varied. Similarly, by adjusting (e.g.,shifting) the set of IFFT inputs to which the DFT output block of size Mis mapped, the frequency-domain position of the transmitted signal canbe adjusted. DFTS-OFDM can be thought of as an OFDM transmission (anIFFT) preceded by a DFT-based pre-coding. Thus, as with OFDM, thespectrum of a DFTS-OFDM signal can be seen as consisting of a number ofsubcarriers.

In some other mobile communication standards, pure OFDM is used insteadof DFTS-OFDM. FIG. 1 b is a block diagram illustrating how pure OFDMworks. Blocks of Mmodulation symbols 111 are applied directly to afrequency mapper 113, which maps the M modulation symbols to selectiveconsecutive inputs of a size N Inverse DFT 115 that can, for example, beimplemented by means of Inverse IFFT processing. By adjusting the blocksize M, the instantaneous bandwidth of the transmitted signal can bevaried. Similarly, by adjusting (e.g., shifting) the set of IFFT inputsto which the DFT output block of size M is mapped, the frequency-domainposition of the transmitted signal can be adjusted. As mentioned above,the spectrum of an OFDM signal can be seen as consisting of a number ofsubcarriers.

SUMMARY OF THE INVENTION

It should be emphasized that the terms “comprises” and “comprising”,when used in this specification, are taken to specify the presence ofstated features, integers, steps or components; but the use of theseterms does not preclude the presence or addition of one or more otherfeatures, integers, steps, components or groups thereof.

In accordance with one aspect of the present invention, the foregoingand other objects are achieved in methods and apparatuses for operatinga network node that serves a plurality of user equipments in a mobilecommunication system. Such operation involves providing the network nodewith a number, N_(RS), of orthogonal reference signals, wherein N_(RS)is less than a maximum number, N_(MAX) _(—) _(AP), of user equipmentantenna ports that can be served by the network node. For each of theantenna ports, a channel quality of a channel between the antenna portand the network node is ascertained. Whenever a number of antenna portsof user equipments being served by the network node exceeds the number,N_(RS), of orthogonal reference signals, all N_(RS) orthogonal referencesignals are allocated to a subset of all of the antenna ports by meansof an allocation process such that: each orthogonal reference signal isallocated to only one of the antenna ports; each antenna port has nomore than one orthogonal reference signal allocated to it; andallocation decisions made by the allocation process are a function ofthe channel qualities of the respective antenna ports such that thehigher the channel quality, the higher priority the correspondingantenna port is given as a candidate for receiving an orthogonalreference signal allocation.

In some embodiments, allocating all N_(RS) orthogonal reference signalsto the subset of all of the antenna ports by means of the allocationprocess is performed such that each of the user equipments has at leastone antenna port to which an orthogonal reference signal is allocated.

In some embodiments, allocating all N_(RS) orthogonal reference signalsto antenna ports by means of an allocation process includes, in around-robin order, allocating one orthogonal reference signal in turn toeach of the user equipments still having an antenna port to which noorthogonal reference signal has yet been assigned, wherein theround-robin order begins with a user equipment associated with a bestascertained channel quality and continues with user equipmentsassociated with ascertained channel qualities in descending order.

In some embodiments, each of the user equipments has a same number ofantenna ports.

In some embodiments, the network node is an eNodeB.

In some embodiments, the network node is a coordination center of adistributed antenna system cell in the mobile communication system.

In some embodiments, each of the antenna ports corresponds to a singleantenna in one of the user equipments.

In some embodiments, each of the user equipments transmits aninformation stream through no more than one of the user equipment'santennas.

In another aspect of embodiments consistent with the invention, acontrol signal is communicated to a user equipment, wherein the controlsignal conveys reference signal allocation information.

In some embodiments, the reference signal allocation informationincludes an indicator that uniquely identifies one or more referencesignals to be used by the user equipment, and an indicator thatassociates each of the one or more identified reference signals with acorresponding one of a number antenna ports of the user equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will be understood byreading the following detailed description in conjunction with thedrawings in which:

FIGS. 1 a and 1 b are block diagrams illustrating how DFTS-OFDM and pureOFDM, respectively, work.

FIG. 2 a illustrates an exemplary subframe for use in the uplink radiointerface of an LTE-type communication system.

FIG. 2 b is an exemplary time/frequency grid illustrating uplinktransmission in an LTE-type communication system.

FIG. 3 is a time/frequency diagram illustrating two exemplary slots with“demodulation reference signals” being transmitted in the fourth longblock of each slot and with a bandwidth equal to the bandwidth of thedata transmission.

FIG. 4 is an illustration of a mobile communication system arrangementin which CMPRX is utilized.

FIG. 5 is a block diagram of an exemplary coordination center capable ofcarrying out various aspects of the invention.

FIG. 6 is, in one respect, a flow chart of steps/processes/functions,carried out by the exemplary coordination center.

FIGS. 7 a and 7 b, in one respect, can be considered to depict a flowchart of steps/processes/functions, carried out by the exemplarycoordination center 501 as part of allocating reference signals to UEantenna ports by means of an allocation strategy that gives higherpriority to antenna ports having a higher quality channel.

FIG. 8 is a block diagram illustrating a control communication from acoordination center to a UE located in the coordination center's DAScell, wherein control information related to reference signalallocations is included as part of the control signaling.

DETAILED DESCRIPTION

The various features of the invention will now be described withreference to the figures, in which like parts are identified with thesame reference characters.

The various aspects of the invention will now be described in greaterdetail in connection with a number of exemplary embodiments. Tofacilitate an understanding of the invention, many aspects of theinvention are described in terms of sequences of actions to be performedby elements of a computer system or other hardware capable of executingprogrammed instructions. It will be recognized that in each of theembodiments, the various actions could be performed by specializedcircuits (e.g., discrete logic gates interconnected to perform aspecialized function), by program instructions being executed by one ormore processors, or by a combination of both. Moreover, the inventioncan additionally be considered to be embodied entirely within any formof computer readable carrier, such as solid-state memory, magnetic disk,or optical disk containing an appropriate set of computer instructionsthat would cause a processor to carry out the techniques describedherein. Thus, the various aspects of the invention may be embodied inmany different forms, and all such forms are contemplated to be withinthe scope of the invention. For each of the various aspects of theinvention, any such form of embodiments may be referred to herein as“logic configured to” perform a described action, or alternatively as“logic that” performs a described action, or alternatively as “meansfor” performing a described action or function.

FIG. 2 is a diagram illustrating a basic subframe structure of an LTEuplink radio interface. It will be appreciated that aspects of the LTEsystem are presented here to facilitate an understanding of variousaspects of the invention. However, characteristics of the LTE systemthat make it a suitable environment for practicing the invention arealso present in other systems (e.g., other OFDM communication systems).Accordingly, the invention is not limited to application only in an LTEsystem, but rather is suitable for use in other communication systems aswell.

The LTE uplink radio interface includes subframes, an exemplary one ofwhich is depicted in FIG. 2 a. Each subframe 200 has a time duration of1 ms, and consists of two equally-sized slots of duration 0.5 ms. As anexample, each slot can consist of seven OFDM symbols. Within one OFDMsymbol, data (e.g., a number, M, of modulation symbols) are transmittedin parallel on a large number of narrowband subcarriers. As is known inthe art, each OFDM symbol includes a cyclic prefix whose purpose is tomake the OFDM signal insensitive to time dispersion on the radiochannel.

The uplink transmission can be described as a time/frequency grid asillustrated in FIG. 2 b, in which each resource element or modulationsymbol corresponds to one subcarrier during one OFDM symbol interval.For an LTE system, the spacing between neighboring subcarriers is 15kHz, and the total number of subcarriers can be as large as 1200. Asalso illustrated in FIG. 2 b, the subcarriers are grouped into resourceblocks, wherein each resource block consists of 12 subcarriers duringone 0.5 ms slot. With seven OFDM symbols per slot, there is thus a totalof 12×7=84 resource elements in a resource block. One such resourceblock is illustrated as the shaded area in FIG. 2 b.

In the LTE radio-access technology, as well as in others, the uplinkradio channel of a mobile-terminal -to-network link can be estimated bymeans of known reference signals that are transmitted by the mobileterminal within specific DFTS-OFDM blocks. The radio channel over abandwidth equal to the instantaneous bandwidth of the uplink datatransmission can be estimated by means of the transmission of so-called“demodulation reference signals”, transmitted within the fourth OFDMsymbol of each slot. Of note is the fact that each demodulationreference signal has a bandwidth equal to the bandwidth of the datatransmission. The situation for two exemplary OFDM slots is illustratedin FIG. 3. These reference signals can, for example, be used for channelestimation for coherent detection of the uplink data transmission fromthe mobile terminal.

The demodulation reference signals can, in the frequency domain, be seenas consisting of a number of subcarriers. Generation of the demodulationreference signals typically is by means of “normal” OFDM processing(i.e., no DFT precoding is used).

Generally speaking, cellular systems suffer from co-channelinterference, whereby simultaneous transmissions use the same physicalresources and thus generate mutual interference. This co-channelinterference reduces the signal quality (e.g., measurable as a signal tointerference plus noise ratio—“SINR”) and in turn reduces the systemcapacity. Systems having a dense deployment of nodes are especiallyinterference-limited, meaning that their performance is limited byco-channel interference.

A technique called “coordinated multipoint reception” (CMPRX) is beingconsidered for use in systems such as LTE-Advanced because it is apromising technology for improving the system-level performance in theuplink direction (i.e., from a user equipment—“UE”—to a base station oreNodeB) in interference-limited scenarios. The basic idea of CMPRX is toallow a baseband receiver to use antennas situated at multiple sites todemodulate the symbols transmitted by various UEs on the uplink. Oneimplementation of CMPRX, illustrated by the arrangement depicted in FIG.4, comprises an eNodeB 401 that is connected (e.g., by means of fibercable 403) to multiple antennas 405 located at different sites. ThiseNodeB 401 acts as a “coordination center” 401 having a geographiccoverage area referred to as a Distributed Antenna System (DAS) cell407. A number of UEs can be present and served by the coordinationcenter 403. In the illustrated embodiment, there are three of them (UE409-1, UE 409-1, and UE 409-3), although it will be appreciated that atany given time there could be more or fewer UEs. In this arrangement,the signals transmitted by each of the UEs 409-1, 409-2, 409-3 in theDAS cell 407 are demodulated together using all of the network antennas405 within the coverage area of the DAS cell 407. It will be appreciatedby those skilled in the art that the present invention will be equallyapplicable to scenarios in which all the receive antennas (i.e.,antennas on the network) belonging to one DAS cell are located at onesite. In such scenarios, the DAS cell becomes an ordinary cell in whichmultiple UEs are allowed to transmit simultaneously on the same set ofsubcarriers.

Two particularly attractive baseband techniques for demodulating thesignals received from the UEs 409-1, 409-2, 409-3 in each DAS cell 407are: successive interference cancellation (SIC) and interferencerejection combining (IRC). Each of these baseband receiver techniquesrequires that the channel between each mobile and each receive antennabe estimated by the uplink receiver. It has been shown that the qualityof these channel estimates greatly influences the performance of SIC aswell as IRC.

As mentioned earlier with reference to FIG. 3, uplink channels aretypically estimated by the uplink receiver from demodulation referencesignals (RSs) that are transmitted from each UE antenna. (Modern UEs areoften designed having two or more transmit antennas to improvetransmitter and receiver performance.) In LTE systems, one OFDM symbolout of each 0.5 ms slot is devoted to the transmission of an RS by allUEs. Hence, when estimating any given uplink channel at the coordinationcenter 401, the other reference signals act as interference, whichdegrades the accuracy of the channel estimate. As the interference amongthe different RSs increases the channel estimation quality decreases. Tomitigate this effect, the reference signals used by all of the UEs beingserved by one DAS cell will ideally be orthogonal with respect to oneanother.

For example, consider a system in which each UE is equipped with N_(tx)transmit antenna ports. The term “antenna port” is used here instead ofthe term “transmit antenna” in recognition of the fact that severalphysical transmitting antennas can be configured such that they appearas one antenna from the perspective of the receiver. The term “antennaport”, then, is intended to cover all possible embodiments, including asingle physical antenna as well as two or more antennas configured toact in concert so as to be equivalent to a single transmitting antennafrom the point of view of a receiver. Note that in the LTE standard,every downlink transmission is always expressed as being carried outfrom a set of antenna ports.

Assume that a number, N_(UE), of UEs can be simultaneously served withinone DAS cell. Thus, ideally, it would be desirable to have(N_(tx)*N_(UE)) orthogonal reference signals available for use in eachDAS cell.

To take a numerical example, consider a DAS cell having 7 antenna sites(which corresponds to 21 sectors). Furthermore, assume that each sectorwill serve a maximum of one UE. This means that the maximum number ofUEs that can be simultaneously served by this DAS cell is equal toN_(UE)=21. If each of the UEs has N_(tx)=2 transmit antenna ports, eachDAS cell would require N_(tx)*N_(UE)=2*21 =42 orthogonal referencesignals. However, current system designs often allocate fewer orthogonalreference signals per DAS cell. For example, the present LTE Release 8standard supports having 8 orthogonal reference signals within each DAScell.

Increasing the number of orthogonal reference signals inherentlyrequires devoting more uplink resources to the transmission of uplinkreference signals, and this in turn reduces the amount of uplinkresources left for transmission of data. This suggests that, ideally,one would like to have as few orthogonal reference signals on the uplinkas possible.

In present systems, the number of uplink orthogonal reference signalsneeded in a DAS cell increases by N_(tx) for every additional UE that isserved by the DAS cell. Hence, with N_(UE) UEs being served by acoordination center, there will be a need to set aside enough uplinkresources to support N_(tx)*N_(UE) orthogonal reference signals. AsN_(UE) and N_(tx) become large, a substantial portion of uplinkresources could be taken away by the reference signals alone, makingthese resources unavailable for transmitting uplink data.

It is therefore desirable to provide methods and apparatuses that alloweach DAS cell to operate with fewer than N_(tx)*N_(UE) orthogonalreference signals while minimally degrading the performance experiencedby each UE within the coordination center's coverage area.

In an aspect of embodiments consistent with the invention, a networknode, such as a coordination center of a DAS cell, assigns all of anumber, N_(RS), of orthogonal reference signals to only a subset of allantenna ports of user equipments being served by the network node. Animplication of this is that, given the presence of a number, N_(TX), oftransmitter antenna ports associated with UEs in the DAS cell, fewerthan N_(TX) transmitted streams will be assigned to the UEs. Unlessotherwise provided for, the coordination center should force each UE totransmit each of its streams from a different one of its availabletransmit antenna ports.

When N_(RS) is less than N_(TX), (i.e., the coordination center hasfewer assignable orthogonal reference signals than there are transmitantenna ports in the DAS cell), an aspect of embodiments consistent withthe invention enables the coordination center to determine the number oforthogonal reference signals that will be assigned to each mobile suchthat the overall performance is degraded as little as possible.

In another aspect of embodiments consistent with the invention, moreorthogonal reference signals (and hence, streams) are assigned totransmit antenna ports having better channel quality than those transmitantenna ports having poorer channel quality. This assignment can besignaled using the downlink control channel to the UEs. This assignmentstrategy is believed to provide advantages because users with poorchannel quality do not benefit much from transmitting multiple streams,and therefore do not experience a substantial degradation in service dueto not being able to transmit as many streams as their number of antennaports would otherwise permit. By contrast, users with good channelquality do benefit greatly by having assigned to them a sufficientnumber of orthogonal reference signals to enable them to make fulleruser of their available transmit antenna ports.

These and other aspects of embodiments consistent with the inventionwill now be described in greater detail.

Beginning first with FIGS. 5 and 6, FIG. 5 is a block diagram of anexemplary coordination center 501 capable of carrying out variousaspects of the invention. FIG. 6 is, in one respect, a flow chart ofsteps/processes/functions, carried out by the exemplary coordinationcenter 501. In another respect, FIG. 6 can be considered to depict thevarious elements of logic configured to carry out the various functionsdescribed in FIG. 6 and its supporting text.

The exemplary coordination center 501 is coupled to receive signalsfrom, and send signals to, each of a number, N_(TRX), of antennassituated at various sites within a DAS cell served by the coordinationcenter 501. The coordination center 501 includes a control unit 503 forcontrolling operations relating to the various functions describedherein. In various embodiments, the control unit 503 may be separatefrom, or alternatively an integral part of, one or more other controlunits (not shown) that control other functions within the coordinationcenter 501.

The coordination center 501 has a number, N_(RS), orthogonal referencesignals available to it (step 601). These may be stored in a table, suchas the table of reference signal assignments 505 illustrated in FIG. 5.In order to facilitate other aspects of the invention, the exemplarytable of reference signal assignments 505 also stores, within the table,information associated with each of the orthogonal reference signalsindicating whether that orthogonal reference signal is available forallocation, and if not, to which antenna port it has been allocated. Ofcourse, in alternative embodiments separate tables may be used to keeptrack of this information.

The coordination center 501 also includes a channel qualitydetermination unit 507, that is logic configured to ascertain a measureof channel quality for each channel existing between a UE's antenna port(“AP”) and the coordination center's own antennas. The control unit 503operates the channel quality determination unit 507 such that a measureof channel quality is determined for each antenna port of the UE'spresently being served by the coordination center 501 (step 603). Any ofa number of well-known channel quality measurements can be made. Forexample, path loss is a useful measurement of quality because it is aslow changing parameter.

The set of channel quality information is made available to the controlunit 503, which then allocates reference signals to the UE antenna portsby means of an allocation strategy that gives higher priority to antennaports having higher quality channels (step 605). (The word “priority” inthis description refers to the right to take precedence and/or precedeothers in obtaining an antenna port allocation.) To facilitate thisaspect in the exemplary embodiment, the control unit 503 generates alist of antenna ports ranked according to their corresponding channelquality, and stores this ranking (list) in an antenna port ranking table509. The generation of a stored list is not essential to the invention,however, and alternative embodiments can be implemented that do notutilize such a table.

Allocation of orthogonal reference signals is made until all of theavailable orthogonal reference signals have been allocated (i.e., thecase in which the number of antenna ports exceeds the available numberof orthogonal reference signals), or until each of the UE antenna portspresently being served by the coordination center have been assigned oneof the coordination center's orthogonal reference signals.

Conditions within the DAS cell change over time: channel qualitychanges, some UEs effectively leave the DAS cell (e.g., by physicallymoving out of the coverage area of the DAS cell or by being turned off),and other UEs effectively enter the DAS cell. Because this is a dynamicsituation, the control unit 503 must periodically determine whether areallocation of reference signals is necessary (decision block 607). Ifnot (“NO” path out of decision block 607), no changes are made. Ifreallocation is necessary (“YES” path out of decision block 607), thenthe process is repeated, beginning with the channel qualitydetermination function (step 603).

A number of allocation strategies satisfying the various principles ofthe invention are possible, and all of these are considered to beembodiments of the invention. It will be appreciated that by basingallocation solely on channel quality, it is possible not only that someUEs will be allocated more orthogonal reference signals for theirantenna ports than other UEs having the same or even more antenna ports.Some UEs could even end up having no orthogonal reference signalsallocated to them at all.

In some alternative embodiments, it is desired to perform the allocationnot only in a way that gives priority to antenna ports having betterchannel quality than to those that don't, but also in a way that ensuresthat each UE is allocated at least one orthogonal reference signal. Anexemplary embodiment of such a strategy utilizes a round-robin approachto orthogonal reference signal allocation, whereby:

-   -   1. The coordination center 501 ranks all of the antenna ports in        decreasing order according to their corresponding channel        quality.    -   2. Beginning at the start of the ranking (i.e., with the UE        whose antenna port is associated with the best quality channel)        and proceeding through to the end of the ranking, the        coordination center 501 assigns one of its available (i.e.,        unassigned) orthogonal reference signals to each of its UEs.    -   3. If any orthogonal reference signals remain unassigned, the        coordination center 501 repeats the process of step 2, starting        at the beginning of the ranking and proceeding through to the        end of the ranking, and making assignments of orthogonal        reference signals to any antenna ports that had not been        previously assigned.    -   4. Step 3 (and by implication, step 2) is repeated until the        coordination center 501 has no more orthogonal reference signals        available for assignment, or until each of the UE antenna ports        being served by the coordination center 501 have been assigned        an orthogonal reference signal.

Aspects of this allocation strategy are now further described inconnection with an embodiment illustrated by FIGS. 7 a and 7 b which, inone respect, can be considered to depict a flow chart of exemplarysteps/processes/functions, carried out by the exemplary coordinationcenter 501 as part of its performance of step 605. In another respect,FIGS. 7 a and 7 b can be considered to depict the various elements oflogic configured to carry out the various functions described in thesefigures and their supporting text.

An initial function of the orthogonal reference signal allocationprocess 605 is to rank antenna ports in descending order from highest tolowest channel quality (step 701). Another initial function is to reseta set of flags, herein referred to as “UE_served” flags (step 703). TheUE_served flags are provided in correspondence to the UEs represented inthe ranking, one flag per UE. When reset, the UE_served flag permits aUE's antenna port to receive an orthogonal reference signal allocation.When set, the UE_served flag prevents the corresponding UE fromreceiving any further orthogonal reference signal allocation.

The orthogonal reference signal allocation process 605 repeatedly loopsthrough the ranking, attempting to assign available orthogonal referencesignals to UE antenna ports until either there are no more availableorthogonal reference signals, or all UE antenna ports have beenallocated one of the orthogonal reference signals. The latter ispossible whenever the actual number of UE antenna ports present in theDAS cell is less than the maximum number for which it was designed.

To begin the looping process, the first antenna port in the ranking(i.e., the one associated with the highest channel quality) is selectedfor use as a candidate antenna port (step 705). The UE_served flag ofthe UE in which the candidate antenna port is located is tested todetermine its state (i.e., set or reset) (decision block 707). In thefirst pass through the loop, it will not be for this highest rankingantenna port (“NO” path out of decision block 707), and one of theavailable orthogonal reference signals is allocated to this candidateantenna port (step 709).

In order to prevent any other antenna ports of the same UE fromreceiving an allocation during this same pass through the allocationloop, the UE's UE_served flag is set (step 711). Also, to prevent thecandidate antenna port from being allocated another orthogonal referencesignal during a subsequent pass through the allocation loop, it isremoved from the ranking (step 713). Alternative ways of handling thisinclude providing another flag for each antenna port, indicating whetherit has received an orthogonal reference signal allocation. Such a flagwould then have to be tested prior to making any allocation to anantenna port.

Next, the ranking is tested to determine whether it is empty (decisionblock 715). This is possible because, as just explained, antenna portsare removed from the ranking once they have received an orthogonalreference signal assignment. If the ranking is empty (“YES” path out ofdecision block 715), the allocation is complete and the process exits(step 723).

However, if the ranking is not empty (“NO” path out of decision block715) it means that more antenna ports are eligible to receive anorthogonal reference signal assignment. Thus, it is further testedwhether there exist any more unallocated reference signals (decisionblock 717). If not (“NO” path out of decision block 717), no furtherallocations are possible and the process exits (step 723).

If there remain unallocated orthogonal reference signals (“YES” path outof decision block 717) a further test is performed to determine whetherthis pass through the loop has reached the end of the ranking (decisionblock 719). If it has (“YES” path out of decision block 719), loopprocessing repeats, starting at the beginning of the ranking In order topermit second (and in subsequent passes of the loop, third, fourth,etc.) antenna ports of UEs to receive an orthogonal reference signalallocation, all of the UE_served flags are again reset (step 703) andthe candidate antenna port is selected as the first antenna portremaining in the ranking (step 705). Processing then continues asdescribed above.

Returning to a discussion of decision block 719, if loop processing hadnot reached the end of the ranking (“NO” path out of decision block719), a candidate antenna port is selected as the next antenna port inthe ranking (step 721). Processing then repeats beginning at the test todetermine whether the UE_served flag of the (new) candidate antennaport's UE has been set (decision block 707). It will be appreciated thatthis is now possible if the candidate antenna port is located in thesame UE as the previous candidate antenna port. If the UE_served flag ofthe candidate antenna port's UE has been set (“YES” path out of decisionblock 707), the candidate antenna port cannot be further considereduntil other antenna ports included in other UEs have first been given achance. Accordingly, processing skips down to the test to determinewhether the end of the ranking has been reached (decision block 719).Processing then continues as earlier described.

The above and equivalent allocation arrangements guarantee that each UEreceives at least one orthogonal reference signal (and one stream).Furthermore, UEs with better channel quality are eligible to have moreorthogonal reference signals (and more streams) assigned than UEs withpoorer channel quality.

In another aspect of embodiments consistent with the invention, once acoordination center has decided how many reference signals should beassigned to each UE (and from which of that UE's antenna ports thereference signals should be transmitted), this information should becommunicated to the UE. This can be done in any number of ways. Forexample, FIG. 8 is a block diagram illustrating a communication from acoordination center 801 to a UE 830 located in the coordination center'sDAS cell 805. In the example, the communication takes place by means ofdedicated control channel 807 on the downlink, although other signalingmechanisms can be used instead. Control information 809 is included aspart of this signaling. The control information 809 includes, but is notlimited to, for example, any of the following: the number of streamsthat the UE 803 is to transmit; the particular reference signals to beused for each of the streams; and which of the UE's antenna ports is touse which reference signal. As the reference signal assignments aredynamic in nature, the UE 803 cannot expect to use these for theduration of its connection in this DAS cell 805. Rather, the UE 803 canbe expressly or inherently (e.g., by means of internal design)instructed to utilize these assignments for a given duration of time, agiven number of frames, or equivalent.

It will be observed that the particular chronological ordering ofinformation illustrated in FIG. 8 is merely for purposes of example, andis not essential to the invention. The invention is considered to beembodied in any type of ordering and/or or encoding of this information.

It will also be appreciated that the various information elementsincluded in the control information 809 can be encoded in any of anumber of ways. For example, reference signals can be directly included,“as is”, in the control information 809. Alternatively, each of thereference signals can be uniquely associated with one of a number ofcodes, in which case it is sufficient to include the corresponding codenumber in the control information 809. Upon receipt, the UE 803 is ableto convert the received code number into the actual reference signalassociated with that code number (e.g., by means of a pre-stored lookuptable). Other information elements in the control information 809 cansimilarly be indicated by means of any of a number of possible encodingschemes, no one of which is essential to the invention.

An advantage of various embodiments consistent with the invention isthat coordination centers/base stations of mobile communication systemsdo not have to be provided with enough orthogonal reference signals topermit an allocation to every antenna port in every possible UE that thecoordination center/base station can serve. This, in turn, reduces theoverhead associated with reference signals on the uplink in suchsystems.

The invention has been described with reference to particularembodiments. However, it will be readily apparent to those skilled inthe art that it is possible to embody the invention in specific formsother than those of the embodiment described above.

For example, the various aspects of the invention have been described inconnection with embodiments utilizing DAS techniques. However, theprinciples illustrated by these exemplary embodiments are alsoapplicable to traditional cell arrangements, in which a single basestation (or equivalent) is associated with only single transmit/receiveantenna.

Thus, the described embodiments are merely illustrative and should notbe considered restrictive in any way. The scope of the invention isgiven by the appended claims, rather than the preceding description, andall variations and equivalents which fall within the range of the claimsare intended to be embraced therein.

1. A method of operating a network node that serves a plurality of user equipments in a mobile communication system, the method comprising: providing the network node with a number, N_(RS), of orthogonal reference signals for uplink signal transmissions, wherein N_(RS) is less than a maximum number, N_(MAX) _(—) _(AP), of user equipment antenna ports that can be served by the network node; and for each of the antenna ports, ascertaining a channel quality of a channel between the antenna port and the network node; whenever a number of antenna ports of user equipments being served by the network node exceeds the number, N_(RS), of orthogonal reference signals, allocating all N_(RS) orthogonal reference signals to a subset of all of the antenna ports by means of an allocation process such that: each orthogonal reference signal is allocated to only one of the antenna ports; each antenna port has no more than one orthogonal reference signal allocated to it; and allocation decisions made by the allocation process are a function of the channel qualities of the respective antenna ports such that the higher the channel quality, the higher priority the corresponding antenna port is given as a candidate for receiving an orthogonal reference signal allocation.
 2. The method of claim 1, wherein allocating all N_(RS) orthogonal reference signals to the subset of all of the antenna ports by means of the allocation process is performed such that: each of the user equipments has at least one antenna port to which an orthogonal reference signal is allocated.
 3. The method of claim 1, wherein allocating all N_(RS) orthogonal reference signals to antenna ports by means of an allocation process comprises: in a round-robin order, allocating one orthogonal reference signal in turn to each of the user equipments still having an antenna port to which no orthogonal reference signal has yet been assigned, wherein the round-robin order begins with a user equipment associated with a best ascertained channel quality and continues with user equipments associated with ascertained channel qualities in descending order.
 4. The method of claim 1, wherein each of the user equipments has a same number of antenna ports.
 5. The method of claim 1, wherein the network node is an eNodeB.
 6. The method of claim 1, wherein the network node is a coordination center of a distributed antenna system cell in the mobile communication system.
 7. The method of claim 1, wherein each of the antenna ports corresponds to a single antenna in one of the user equipments.
 8. The method of claim 7, wherein each of the user equipments transmits an information stream through no more than one of the user equipment's antennas.
 9. The method of claim 1, comprising: communicating a control signal to a user equipment, wherein the control signal conveys reference signal allocation information.
 10. The method of claim 9, wherein the reference signal allocation information includes: an indicator that uniquely identifies one or more reference signals to be used by the user equipment; and an indicator that associates each of the one or more identified reference signals with a corresponding one of a number antenna ports of the user equipment.
 11. An apparatus for controlling a network node that serves a plurality of user equipments in a mobile communication system, the apparatus comprising: a processor configured to execute logic; and at least one computer readable carrier that stores logic executable by the processor, the computer readable carrier comprising: logic configured to provide the network node with a number, N_(RS), of orthogonal reference signals for uplink signal transmissions, wherein N_(RS) is less than a maximum number, N_(MAX) _(—) _(AP), of user equipment antenna ports that can be served by the network node; and logic configured to ascertain, for each of the antenna ports, a channel quality of a channel between the antenna port and the network node; logic configured to allocate all N_(RS) orthogonal reference signals to a subset of all of the antenna ports by means of an allocation process such that, whenever a number of antenna ports of user equipments being served by the network node exceeds the number, N_(RS), of orthogonal reference signals: each orthogonal reference signal is allocated to only one of the antenna ports; each antenna port has no more than one orthogonal reference signal allocated to it; and allocation decisions made by the allocation process are a function of the channel qualities of the respective antenna ports such that the higher the channel quality, the higher priority the corresponding antenna port is given as a candidate for receiving an orthogonal reference signal allocation.
 12. The apparatus of claim 11, wherein the logic configured to allocate all N_(RS) orthogonal reference signals to the subset of all of the antenna ports by means of the allocation process operates such that: each of the user equipments has at least one antenna port to which an orthogonal reference signal is allocated.
 13. The apparatus of claim 11, wherein the logic configured to allocate all N_(RS) orthogonal reference signals to antenna ports by means of an allocation process comprises: logic configured to allocate, in a round-robin order, one orthogonal reference signal in turn to each of the user equipments still having an antenna port to which no orthogonal reference signal has yet been assigned, wherein the round-robin order begins with a user equipment associated with a best ascertained channel quality and continues with user equipments associated with ascertained channel qualities in descending order.
 14. The apparatus of claim 11, wherein each of the user equipments has a same number of antenna ports.
 15. The apparatus of claim 11, wherein the network node is an eNodeB.
 16. The apparatus of claim 11, wherein the network node is a coordination center of a distributed antenna system cell in the mobile communication system.
 17. The apparatus of claim 11, wherein each of the antenna ports corresponds to a single antenna in one of the user equipments.
 18. The apparatus of claim 17, wherein each of the user equipments transmits an information stream through no more than one of the user equipment's antennas.
 19. The apparatus of claim 11, comprising: logic configured to communicate a control signal to a user equipment, wherein the control signal conveys reference signal allocation information.
 20. The apparatus of claim 19, wherein the reference signal allocation information includes: an indicator that uniquely identifies one or more reference signals to be used by the user equipment; and an indicator that associates each of the one or more identified reference signals with a corresponding one of a number antenna ports of the user equipment. 